Blackening process

ABSTRACT

TELEVISION TUBE APERTURE MASKS AND SHADOW MASKS MADE OF STEEL ARE BLACKENED WITH AN ADHERENT OXIDE COATING BY HEATING THE MASKS IN AN OXIDIZING ATMOSPHERE MADE PRINCIPALLY OF COMBUSTION GASES. THE COMPOSITION OF THE OXIDIZING ATMOSPHERE IS CONTROLLED TO PROVIDE A MINOR AMOUNT OF FREE OXYGEN AND A MINOR AMOUNT OF LIVE STEAM   D R A W I N G

Jan. 2,1973 N. s. DALLAS ET AL BLACKENING PROCESS Filed Feb. 2, 1970 Jan. 2, 1973 N. s. DALLAS ET AL 3,708,351

, y BLACKENING PnocEss Filed Feb. 2. 1970 2 sheets-sheet 2 nited States Patent Oce 3,708,351 Patented Jan. 2, 1973 U.S. Cl. 14S-6.35 6 Claims ABSTRACT F THE DISCLOSURE Television tube aperture masks and shadow masks made of steel are blackened with an adherent oxide coating by heating the masks in an oxidizing atmosphere made principally of combustion gases. The composition of the oxidizing atmosphere is controlled to provide a minor amount of free oxygen and a minor amount of live steam.

SUMMARY OF THE INVENTION Aperture masks and shadow masks made of steel are conventionally used in the manufacture of television receivers. To minimize internal reflection in the receiver it is necessary to blacken such pieces, and it has been the practice to blacken them by oxidation at elevated temperatures in a furnace chamber in combustion gases derived from the air combustion of natural gas.

It is desired that the oxidation process produce uniform, strongly adherent coatings on the aperture masks and shadow masks, but in practice the coatings have often proved to be non-uniform and subject to aking.

In accordance with this invention, the oxidation process is controlled to provide in the combustion gas contacted with the masks a controlled amount of free oxygen and live steam. Specifically, the gas introduced into the furnace chamber is controlled in composition to contain at least about 1/z% by Volume of oxygen, and preferably from about 1% to about 2%, and at least about 1/2% by volume of added live steam, and preferably from about 1% to about 2% thereof. The gas should also contain from about 2% to about 7% of combustible gas (carbon monoxide and hydrogen, combined) and preferably from about 41/2 to about 61/2 thereof.

It is possible to approach the desired composition in the combustion gas by careful selection of the natural gas fuel used in the gas generation process along with control of the air to gas ratios and combustion conditions therein. In most instances, however, air and live steam are added to the natural gas combustion product after completion of the combustion in order to obtain the desired final composition.

The combustion gas of the desired composition is preferably introduced into the blackening furnace chamber at a midpoint thereof so that the freshest gas with the highest oxygen and steam content comes into contact with fully heated masks.

The steam content of the combustion gas should be at a level such that the gas is not below its dew point even at the coolest part of the furnace. Otherwise condensation on the masks can occur with consequent unevenness of the coatings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF 'I'HE DRAWING The invention may be better understood by reference of the drawings in which:

FIG. 1 is a partially schematic side elevation of the blackening apparatus;

FIG. 2 is a side elevation in cross section of the furnace of FIG. 1;

FIG. 3 is a transverse view of the furnace in cross section, taken along plane 3-3 of FIG. 2;

FIG. 4 is a plan View of the furnace in cross section, taken along plane 4--4 of FIG. 2;

FIG. 5 is an enlarged perspective View of the entrance curtains used to seal the furnace ends; and

FIG. 6 is an enlarged perspective view of a gas distribution manifold in the furnace.

DETAILED DESCRIPTION As may be seen in the drawings, the blackening apparatus comprises a mesh belt 11 on which the pieces to be blackened 12 are mounted to move successively through entrance tunnel 13, furnace 14 and exit tunnel 16. As pieces 12 enter entrance tunnel 13 they pass a series of entrance curtains 17a, 17b, 17c and 17d, each made of a plurality of hanging stainless steel strips 18 which flex to allow entrance of the pieces to be blackened and immediately recover to keep the tunnel end sealed. A series of electrical heating elements 19, situated in the upper and lower portions of the furnace provide heat to the pieces to be blackened and to the gases within the furnace.

Combustion gas is generated in gas generator 21 by the combustion of natural gas in air and is generally cooled to about ambient temperature to remove most of its moisture before reaching feed line 22. Air, or oxygen, is introduced through line 23 to produce the desired oxygen content in the gas, as discussed in more detail below, and the combined gas passes to the side of the furnace where live steam is added through line 24. The combined gas stream is split in two, with a portion going directly into the furnace through manifold 26 below belt 12 and the remainder passing through branch line 27 to manifold 28 above the belt. Manifold 26 contains a plurality of apertures in two parallel rows spaced apart and directed upward, and manifold 28 has similar rows of apertures directed downward.

As the pieces pass through furnace 14, they pass into exit tunnel 16, the downstream portion of which has a cooling jacket 29. Cooling water passes through jacket 29, being fed thereto through line 31 and withdraws through line 32. A series of exit curtains 33a, 33h, 33C and 33d, are similar in construction and function to entrance curtains 17a-17d.

The oxidizing gas introduced to the middle of furnace 14 through manifolds 26 and 28 moves toward the opposite ends thereof with a portion going through entrance tunnel 13 before passing out through the entrance curtains and with the remainder going through exit tunnel and being cooled before passing out through the exit curtains. Exhaust tunnel 34 at the entrance end and exhaust tunnel 36 at the exit end draw olf the exhaust oxidizing gas, in each case blending it with ambient air from outside the curtains.

Gas generator 21 is of known construction and design and does not per se constitute part of this invention. It comprises air and gas flow meters, a proportioning mixer, a compressor, a fire check, a burner, a combustion chamber, and a cooler with a condensate separator and trap.

Combustion in gas generator 21 is generally carried out at air/natural gas ratios between 8:1 and 10:1. In this generator air/ gas ratios in excess of 10:1 tend to produce temperatures too high for the burner to handle with safety, and air/gas ratios below 8:1 tend to raise the hydrogen concentration of the combustion gas to the lower explosive limit and the carbon monoxide concentration to the point where it can be dangerous to health. With other kinds of burners and combustion chambers and with proper safety precautions air/gas ratios outside 0f the above limits may be employed.

The cooler u'sed in gas generator 21 uses water to cool the hot combustion gas and is preferably operated to cool the gas to about 70 F. rather than to a higher temperature which would deposit lime in the water lines. When the combustion gas is cooled to this low temperature, it loses most of its moisture content; and a portion thereof must be replaced through line 24 to obtain optimum oxidation in the blackening apparatus.

In a typical combustion at an air/ gas ratio of :1 the composition of the product gas (by volume), after removal of most of the water vapor through cooling is as follows:

oxidizing gases Carbon dioxide.- Reducing gases (combustibles) .{Hydrogen l to 3%: Inert pas Nitrogen. 80 to 86%.

Ais is added to this combustion gas through line 23 This gas is introduced into furnace 14 at a midpoint thereof through manifolds 26 and 28. It has been found that best results are obtained when the pieces to be blackened are not brought into contact with the freshest oxidizing gas until the pieces are brought up to a minimum elevated temperature of the order of about 250 F.

The pieces are loaded onto belt 11 prior to its entry into entrance tunnel 13. As the pieces pass through curtains 17a-17d, the curtains close behind each piece to help keep the oxiding gas of the desired composition within the oxidizing zone and to keep ambient air out.

The time for passage of the pieces through the entrance tunnel, furnace and exit tunnel is a function of the weight and configuration of the parts to be blackened. In a typical operation, thirty minutes of process time may be required.

The process is applicable to iron-containing articles other than television tube aperture masks and shadow masks, and works well with both low carbon and high carbon steels. It may be used, for example, for blackening steel elements used in vacuum tubes.

The temperature in the furnace is suitably maintained between about 1100 F. and 1200 F. and cooling in the exit tunnel may bring the temperature of the pieces down to about 250 F. to 300 F. at curtain 33a, and to lower temperatures at curtain 33b.

The amount of oxygen is generally maintained below about 3%, although 1 to 2% is preferred.

With careful control of the combustion process which generates the gas and the degree of cooling immediately thereafter, it is possible to eliminate the addition of external live steam, provided that the lines between the gas generator and the furnace are either insulated or heated so that condensation in the lines is avoided. However, the addition of live steam from an external source is preferred.

It will be understood by those skilled in the art that other modifications and variations of this invention may be employed without departing from the scope of the appended claims.

What is claimed is:

1. The method of blackening an iron-containing article by treatment of said article in an atmosphere of oxidizing gas comprising:

(a) heating the article to be blackened to a minimum temperature of about 250 F.;

(b) supplying to an oxidizing zone an oxidizing gas comprising a partially combusted gas including live steam at 1% to 2% by volume and oxygen ranging from 1/z% to 3% by volume;

(c) subjecting said article to oxidizing gas in a temperature in the range of 1,l00 F. to 1,200 F. to thoroughly blacken said article.

2. The method of claim 1 wherein said oxidizing gas contains from about 2% to about 7% by volume of combustible gas comprising hydrogen and carbon monoxide.

3. The method of claim 1 wherein said iron-containing articles are continuously passed through said oxidizing zone and wherein said oxidizing gas is introduced into said oxidizing zone at a midpoint thereof.

4. The method of claim 1 wherein said iron-containing articles are cooled from said elevated temperature while still in contact with said oxidizing gas.

5. The method of claim 1 wherein said articles are cooled to a temperature not higher than about 300 F. before leaving said oxidizing zone.

6. The method of claim 5 wherein said iron-containing articles are television tube aperture masks.

References Cited UNITED STATES PATENTS 2,269,943 1/ 1942 Kser 14S-6.35 X 2,543,710 2/1951 Schmidt et al 148-635 1,346,473' 7/1920I Swan 14S-6.35 1,318,027 10/1919 Thompson 14S-6.35

RALPH S. KENDALL, Primary Examiner C. WESTON, Assistant Examiner U.S. C1. X.R. 117-106 R 

